1. Field of the Invention
The present invention relates to a defect-signal generating circuit and an optical disk reproducing device having the same.
2. Description of the Related Art
Recently, optical disks have been widely employed as a large-capacity recording medium capable of recording mass data. Data recorded in such optical disks has been reproduced using an optical disk reproducing device.
More specifically, first, the conventional optical disk reproducing device casts reading light upon an optical disk, generates an RF signal using an optical pickup from the reflected light thereof, generates an input signal based on this RF signal, and then reproduces the data recorded in the optical disk by an signal processing circuit processing this input signal.
Now, in the event that there is a scratch or soiling or the like on the optical disk, a dark defect markedly reducing the amount of reflected light from the optical disk or a bright defect markedly increasing the amount of reflected light from the optical disk sometimes occurs, and consequently, data cannot be correctly reproduced in some cases due to this dark defect or bright defect.
In light of this, the conventional optical disk reproducing device includes a defect-signal generating circuit for generating a defect signal indicating a defect on an optical disk, and reproduces data correctly by performing servo control of the optical pickup such as focus control, tracking control, and so forth based on this defect signal (see Japanese Unexamined Patent Application Publication No. 2000-90446, for example).
As illustrated in FIG. 9, this defect-signal generating circuit 100 comprises a peak-holding circuit 101, dark-defect detection circuit 102, bright-defect detection circuit 103, and a logical-OR computing circuit 104.
The dark-defect detection circuit 102 comprises a dark-defect peak-holding circuit 105, dark-defect level-adjusting circuit 106, and dark-defect comparator 107. The bright-defect detection circuit 103 comprises a bright-defect peak-holding circuit 108, bright-defect level-adjusting circuit 109, and bright-defect comparator 110.
With the defect-signal generating circuit 100 thus configured, as illustrated in FIGS. 9 and 10, the peak-holding circuit 101 is configured so as to follow up an input signal S100 quickly, and output a peak-hold signal S101 changing at approximately the same time as the input signal S100.
On the other hand, the dark-defect and bright-defect peak-holding circuits 105 and 108 are configured so as to have a longer time constant than the peak-holding circuit 101 and slow signal follow-up, and output a dark-defect intermediate signal S102 and bright-defect intermediate signal S103 respectively, which change more sluggishly than the peak-hold signal S101.
Also, the dark-defect level-adjusting circuit 106 generates a dark-defect threshold signal S104 which is adjusted so that the signal level of the dark-defect intermediate signal S102 is lower than that of the peak-hold signal S101 except for a portion originating a dark defect. On the other hand, the bright-defect level-adjusting circuit 109 generates a bright-defect threshold signal S105 which is adjusted so that the signal level of the bright-defect intermediate signal S103 is higher than that of the peak-hold signal S101 except for a portion originating a bright defect.
Also, the dark-defect comparator 107 outputs a dark-defect signal S106 which changes to “High” level when the signal level of the peak-hold signal S101 is below the signal level of the dark-defect threshold signal S104. On the other hand, the bright-defect comparator 110 outputs a bright-defect signal S107 which changes to “High” level when the signal level of the peak-hold signal S101 is above the signal level of the bright-defect threshold signal S105.
Also, the logical-OR computing circuit 104 outputs a defect signal S108 which stays at the “High” level while detecting a dark defect and a bright defect by carrying out the logical OR operation between the dark-defect signal S106 and the bright-defect signal S107.
The optical pickup enters a servo-holding state in which operation of the servo is stopped while the defect signal S108 output from the defect-signal generating circuit 100 is at the “High” level.
Subsequently, when the defect signal S108 is at the “Low” level again, the servo-holding state is released. This is how the conventional optical disk reproducing device handles a defect on an optical disk such as a dark defect and a bright defect and the like.
That is to say, when generating the defect signal S108, the conventional optical disk reproducing device, as illustrated in FIG. 10 and FIGS. 11A and 11B, generates the defect signal S108 by comparing the peak-hold signal S101 for following up the input signal S100 quickly with the dark-defect threshold signal S104 and bright-defect threshold signal S105 for following up a signal slower than this peak-hold signal S101 (see Japanese Unexamined Patent Application Publication No. 2000-90446, for example).
Thus, when generating the defect signal S108, the conventional optical disk reproducing device having the defect-signal generating circuit 100, as illustrated in FIG. 10 and FIGS. 11A and 11B, generates the defect signal S108 by comparing the peak-hold signal S101 for following up the input signal S100 quickly with the dark-defect threshold signal S104 and bright-defect threshold signal S105 for following up a signal slower than this peak-hold signal S101.
Consequently, as illustrated in FIG. 11A, while a dark defect occurs, the signal level of the dark-defect threshold signal S104 (solid line in FIG. 11A) gradually drops with some delay following the signal level of the peak-hold signal S101 dropping (dashed line in FIG. 11A), and then becomes equal to the signal level of the peak-hold signal S101 in the portion indicating that the dark defect is still occurring.
In other words, the signal level of the peak-hold signal S101 rises above that of the dark-defect threshold signal S104 regardless of the dark defect continuing, so the defect signal S108 is unintentionally changed from the “High” level to the “Low” level in some cases.
Similarly, as illustrated in FIG. 11B, while a bright defect occurs, the signal level of the bright-defect threshold signal S105 (solid line in FIG. 11B) gradually rises with some delay following the signal level of the peak-hold signal S101 rising (dashed line in FIG. 11B), and then becomes equal to the signal level of the peak-hold signal S101 in the portion indicating that the bright defect is still occurring.
In other words, the signal level of the peak-hold signal S101 falls below that of the bright-defect threshold signal S105 regardless of the bright defect continuing, so the defect signal S108 is unintentionally changed from the “High” level to the “Low” level in some cases.
Thus, upon the defect signal S108 changing from the “High” level to the “Low” level regardless of the dark defect and bright defect still continuing for some time, unintended servo control is performed based on the erroneous input signal S100 at the time of the dark defect and bright defect. Consequently, there is the possibility that the servo cannot be controlled in a stable manner at the time of releasing the servo-holding state, resulting in data being not able to be reproduced correctly.